Thermosetting epoxy polymers exhibit excellent stiffness and strength due to their high cross-linked density and are commonly utilized as matrices to make fiber reinforced composites. However epoxy thermosets are brittle and typically possess a low fracture toughness that restricts their applications. One of the promising mechanisms of toughening of epoxy thermosets is an integration of micro- and nano-scale fillers. Among these nano-fillers, MXenes, a recently discovered family of 2D transition-metal carbides, can be used to produce multifunctional polymer nanocomposites due to their excellent electrical, thermal, magnetic, and mechanical properties as well as 2D morphology. Density functional theory and coarse-grained molecular dynamics simulations are used to predict the structural properties of Ti2C(OH)2 and Ti3C2(OH)2 MXenes and epoxy interface from the first principles and to elucidate mechanisms of microscopic fracture of the composite under uniaxial tension, respectively. Our results suggest that the structure of epoxy and the binding energy are largely unaffected by MXene type (Ti2C(OH)2 or Ti3C2(OH)2). Binding between Ti3C2(OH)2 and epoxy becomes stronger with less hydrogen coverage of Ti3C2(OH)2 surface and reaches the maximum at 6% coverage due to increase in electrostatic interactions. The simulation results show that the Young’s modulus of MXene/epoxy composite is greater compared to pure epoxy which originates from the stress transfer between the matrix system and the nano-filler, where the modulus linearly increases with the filler loading up to 1 vol. %. At higher filler contents, the increase of the modulus is slowed down due to filler aggregation. Our results show MXene/epoxy composite will fail at lower strain compared to pure epoxy due to the onset of nano-scale void formation in the resin at stress concentration sites along the edge of the MXene particles.. Although, the accuracy of our results is subject to the length and time scales limitations of modeling, our results are in qualitative agreement with experiments, which also detected the existence of voids in the composite near the edges of MXene flakes.

Computational study of MXene/epoxy nanocomposite interface and mechanical properties

2019 Fall National Meeting

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